The present invention relates in general to a flow molding process and associated apparatus. More particularly, the invention pertains to the flow molding of either two-dimensional or three-dimensional parts or items constructed of plastic or the like material. The techniques of this invention may be used in constructing parts for use in many different fields such as in the automotive industry in, for example, the manufacture of decorative interior plastic parts.
The conventional molding technique for plastic parts involves a heating of the mold cavity in combination with a closing of the mold cavity to compress the plastic into the desired configuration. The known techniques, such as shown in FIG. 1 of this application as discussed in more detail hereinafter, are adequate in constructing some parts or items, however, these techniques are generally only useful in constructing substantially planar pieces hereinafter termed two-dimensional pieces. By two-dimensional pieces, we are not referring to pieces that have no third dimension but are referring to pieces that either have a very small third dimension or pieces that depart in their construction, only a slight amount from a planar configuration. In the molding of three-dimensional pieces, and especially those requiring an accurate imprinting, these traditional techniques are not adequate, as it has been found that the plastic tends to burn in some areas and yet remain cold and thus not flow in other areas. This occurs because the heat gradient is not uniform at the plastic surface at all points therealong. A plastic piece such as shown in the illustration of FIG. 3 is considered as a three-dimensional part and the usual prior art technique in constructing this part is to convert a two-dimensional part into the three-dimensional part by reheating and stretching and bending the part over a mandrel. However, this technique results in a loss of distortion of the grain or pattern of the part and, in addition, internal stresses are produced which may cause material fatigue and a loss of shape of the item.
In some prior techniques, where the plastic is compressed by means of the interacting motion between the mold halves, their tends to be created an unequal pressure gradient on the plastic when constructing three-dimensional parts such as shown in the illustration of FIG. 3, discussed hereinafter. One purpose of the present invention is to provide an improved process overcoming this problem.
Pertinent prior art is found in the related applications referred to herein including such patents as the U.S. Pat. to Brown, No. 2,388,824. Although this patent teaches the use of parallel electrodes, the plastic piece that is being formed is generally of two-dimensional type, as defined herein and not of three-dimensional type. As indicated previously, in molding three-dimensional pieces, such traditional techniques are not adequate primarily because of the uneven temperature gradient that is created.
In view of these prior art disadvantages, it is an object of the present invention to provide an improved flow molding technique, particularly useful for molding three-dimensional parts wherein a uniform temperature gradient may be established across the part to prevent any burning or cold spots in the final part.
Another object of the present invention is to provide an improved flow molding technique particularly useful for molding three-dimensional parts and wherein the plastic has applied thereto, substantially equal pressure along all surfaces thereof assuring a good imprint from the mold along with the application of an equal temperature gradient throughout the plastic material which assures an even flow of the material.
Another object of the present invention is to provide a flow molding process and associated apparatus wherein the molds are of simple and inexpensive construction especially in comparison to injection molding apparatus.
A further object of the present invention is to provide a flow molding technique employing a high radio frequency field for heating primarily the plastic within the mold and which is characterized by a reduced cool down time and a reduced amount of energy consumption.